Lithotripters are medical devices designed to fragment upper urinary tract stones, gall stones, renal calyceal stones, renal pelvic stones, and upper ureteral stones. Fragmentation of a stone in a patient is effected by focusing shock waves on the stones. Focusing is often achieved using an ellipsoidal reflector that is designed to reflect shock waves originating at one focus of the reflector to the other focus of the reflector which is coincident with the stone. Other types of focusing, such as a spherical array of piezoelectric transducers are also known.
A basic requirement in all types of lithotripters is to accurately position a patient relative to the lithotripter such that the stone to be fragmented is positioned precisely at the focus of the reflector. Mispositioning is physiologically damaging to the patient, and is to be avoided as much as possible. The problem, of course, is that the stone is internal to patients and its location cannot be determined by visual inspection. Thus, either X-ray based observations, in the case of kidney stones which are substantially opaque to X-ray radiation, or ultrasound scanning in the case of gall stones which are substantially transparent to X-ray radiation, must be used. When X-ray observation is employed, it is conventional to establish the location of a stone by using a fluoroscope in the form of a mobile C-arm carrying an X-ray source on one end of the arm and an image intensifier on the other end. The C-arm is physically associated with the lithotripter in that the focus of the reflector of the lithotripter has a fixed location in the display produced by the image intensifier. A patient is positioned on a treatment table between the X-ray source and the image intensifier. By viewing the stone in the display produced by the image intersifier, the table can be moved relative to the lithotripter until the stone is positioned in the display at a location coincident with the focus of the lithotripter reflector. After exact positioning is achieved, treatment by the lithotripter can be carried out.
In order to avoid intrusive imaging of a patient by X-rays, and especially in the case of treating gall stones which are transparent to X-rays, ultrasound imaging is currently being employed to locate a stone to be fragmented. The use of ultrasound for imaging purposes introduces a degree of complexity to the procedure described above because of the necessity for acoustically coupling the ultrasound transducer to the patient in order to achieve reasonable images, a complexity that is not present in X-ray imaging. While several techniques are presently available for using ultrasound imaging in a lithotripter environment, each technique has its unique deficiency. For example, one system in use utilizes an ultrasound transducer clamped in a fixed position relative to the shock wave generator means. This requires the transducer to be in direct contact with a patient during treatment to secure the required acoustical coupling with a patient. As a consequence, adjustment must be made for different patients because the depth of a stone in the body of the patient varies from patient.
In another system disclosed in U.S. Pat. No. 4,890,503, the ultrasound scanner is built into a spherical array of piezoelectric transducers with an ultrasound transducer being located at the center thereof. This permits a calibration process to be carried out prior to treatment wherein both the array of piezoelectric transducers and the ultrasound transducer are moved until the stone in a patient is located at the focus of the array. After this, the shockwave generator is activated.
One of the problems with this arrangement is the complexity of the equipment, and particularly maintenance or replacement of the ultrasound transducer which idles the equipment. In addition, this arrangement does not permit a technician to utilize the same ultrasound probe for both examination of the patient and for positioning the patient prior to and during treatment.
Other systems are known wherein an ultrasound transducer is located in a multi-jointed arm provided with position sensors at the joints by which the spatial location of the ultrasound transducer is determined. From such location, and with an ultrasound scan, the position in space of a stone in a patient is computed relative to the focus of the shock wave generator. The patient and/or generator are then manipulated so that the stone and focus coincide. Obviously, this is a very complicated system and, as a consequence, is very expensive.
It is therefore an object of the present invention to provide new and improved lithotripter, and a method for using the same, which is less complicated than the apparatus of the prior art, and which ameliorates the deficiencies therein.